1. Technical Field
This invention relates to portable telephones arranged for operation in wireless communication systems and more particular to a portable telephone arranged for operation in dissimilar wireless communication systems.
2. Description of the Prior Art
Portable telephones are used in a variety of wireless communication systems. These communication systems include cellular radio systems and present-day cordless telephone systems which are growing in popularity; both having achieved broad customer acceptance. Both of these systems have different protocol interface requirements, however.
Various types of cellular radio systems have been described which provide radio telephone service to a large number of mobile subscribers using a relatively small number of frequencies. Such service is provided by dividing the service area into a number of cells and reusing the frequencies in non-adjacent cells. Once such system is described in The Bell Systems Technical Journal, Volume 58, January 1979, Number 1, particularly in papers entitled "Advanced Mobile Phone Service: Introduction, Background and Objectives" by W. R. Young and "The Cellular Concept" by V. H. MacDonald.
Although a cellular radio system is capable of satisfactorily providing telephone service to portable telephones over a wide area, a portable telephone operating in such system has to perform a number of complex procedures. These include registering with the closest cellular base station and communicating with this base station which, in turn, is arranged to receive, set-up or hand-off a telephone call. The base station equipment also has to perform a number of complex procedures for carrying out these functions in the cellular radio system. Thus telephone service in a cellular radio system is complex and, for the present, expensive.
Cordless radio systems have been available in the an for some time. One such system was first described in, for example, Telephony, Jul. 22, 1967, pages 38, 54 and 55, in an article entitled "Bell Labs is Developing New Lineless Telephone". In this article, the lineless telephone is described as performing the major functions or a regular telephone set, to wit, the unit connects with the telephone network via a radio link to a fixed station. This fixed station, in turn, is connected to a telephone line or extension line. This lineless telephone also provides simultaneous two-way conversation, as well as supervision (connecting and disconnection), dialing and ringing.
Present-day cordless telephone arrangements, such as is described in U.S. Pat. No. 4,706,274 which issued to William Baker et at. on Nov. 10, 1987 and U.S. Pat. No. 5,044,010 which issued to Richard Frenkiel et al. on Aug. 27, 1991, have evolved from this basic wireless telephone system. These cordless telephones provide cordless telephone service to the network in a less complex manner than the cellular telephones provide cellular telephone service. Also, because of the simpler circuitry employed in cordless telephone arrangements, cordless telephone service is considerably more economical to date than cellular radio service. The presently available cordless telephones, however, are not capable of providing communications over a range that extends much beyond one's residence or office where an associated base for the cordless telephone is located. Also, there are only ten presently allocated communication channels (25 channels have been proposed) in the 46-49 MHz range in the United States, for example, over which this type cordless telephone may operate. Thus, not only is there a limitation of insufficient operating range, there is also the potential problem of locating an available or free channel with these telephones as they continue to increase in popularity. Moreover, voice privacy also suffers in that others may move to and monitor a channel occupied by a user of this type of cordless telephone.
In an effort to obtain the beneficial aspects of both cordless and cellular service, one arrangement described in U.S. Pat. No. 4,989,230 provides a cellular-cordless telephone in which a cellular transceiver and a cordless transceiver are combined in a single housing. Although this arrangement provides for improved wireless service by using both cordless and cellular transceivers, the cordless transceiver of this arrangement has the above described limitations of insufficient operating range, limited channel availability and lack of user privacy.